Heterojunction bipolar transistors (HBTs) are expected to play a large role in the future microwave and millimeter-wave power applications. The structure of a conventional HBT is depicted in FIG. 1. The HBT has a semi-insulating GaAs substrate, a n.sup.+ -GaAs subcollector layer, a n.sup.- -GaAs collector layer, a p.sup.+ -GaAs base layer, a n-AlGaAs or GaInP emitter layer, and a n.sup.+ -GaAs emitter cap layer. Because the HBTs have high speed, high current handling capability and high power efficiency, microwave power amplifiers using the AlGaAs/GaAs or GaInP/GaAs HBTs have demonstrated excellent performance.
However, there are two major drawbacks in using the HBTs as power transistors in these microwave applications, particularly when a large number of the HBTs are used. The current gain of an HBT in the prior art has a temperature dependence of .sup..about. exp(.DELTA.E.sub.V /kT), where .DELTA.E.sub.V is the valance band offset energy of the heterojunction, i.e., the current gain decreases with the increase of junction temperature. This behavior together with self-heating effect results in a negative differential resistance (NDR) in an I-V curve. As a consequence, when the junction temperature rises, the performance of the HBT deteriorates.
The second problem is related to the current gain collapse at a high operation bias. When a multi-finger HBT device is operated at a high current level to generate a large power output, the device's thermal property becomes unstable, and ultimately the current gain of the multi-finger HBT device collapses. FIG. 2 displays as an example I-V curve of a conventional two-finger GaInP/GaAs HBT device. As the operation bias is increased, the thermal effect causes the current gain to collapse.
The problems of the NDR and the current gain collapse are related to the emitter structure of the HBT. According to W. Liu et al.'s article, "Temperature dependence of current gains in GaInP/GaAs and AlGaAs/GaAs heterojunction bipolar transistors," IEEE Trans. Electron Device, vol. 40, No. 7 (1993), pp. 1351-1353, the gain degradation with increasing temperature for a conventional HBT is largely attributed to the structure limitations in which the value of .DELTA.E.sub.V is not high enough and the hole current injected from the base to the emitter is dominated by diffusion.
In the prior art, the problems were overcome by using an emitter ballast resistor. See, for example, G. B. Gao et al., "Emitter ballasting resistor design for, and current handling capability of AlGaAs/GaAs power heterojunction bipolar transistors," IEEE Trans. Electron Device, vol. 38, No. 2 (1991), pp. 185-197. However, the ballast resistor results in the power gain reduction and degrades the RF performance. Recently, U.S. Pat. No. 5,389,554, granted to W. Liu et al. on Feb. 14, 1995, discloses another method to increase the valence band offset by changing aluminum composition in an AlGaAs/GaAs system so as to prevent the NDR. However, changing material composition is only applicable to the AlGaAs/GaAs system because of lattice mismatch with other materials.
Alternatively, W. Liu et al.'s article, "The use of base ballasting to prevent the collapse of current gain in AlGaAs/GaAs heterojunction bipolar transistors," IEEE Trans. Electron Device, vol. 43, No. 2 (1996), pp. 245-251, discloses the use of base ballast to prevent the gain collapse. However, in order to maintain the RF performance, this technique requires a larger capacitance which costs the chip area thereby reducing the efficiency of integration of the HBTs.